The very high velocity air flowing into the air intake of a jet propulsion engine such as a gas turbine engine of an aircraft, is a potential source of power. In the aircraft engines which are at present in use however, this power is not utilised in developing the propulsive power of the engine.
Furthermore, it may be found that, during a major part of an aircraft flight, the aircraft is moving at such a speed that the volume of air taken into the jet propulsion engine exceeds that which can be accepted by the first stage of the engine air compressor system. The result of this is deceleration of the air on impact with the air intake and the front of the compressor rotor, and this causes a negative thrust along the line of flight.